Illumination devices that are very low power consuming devices are used in many applications, such as vehicles, household applications, indicators, data communication, and applications involving usage of light source. Examples of these devices non-exhaustively include Light Emitting Devices (LEDs) and Light Amplification by Stimulated Emission of Radiation (LASER) devices. These low power illumination devices are also used as light source in Time-of-Flight (ToF) imaging applications. ToF cameras work on a principle of illuminating an object by a light source and detecting light reflected from the illuminated object. Further, an image of the illuminated object is constructed based on a phase difference between the light emitted by the illumination device and the light reflected from the illuminated object.
There are stringent requirements for precise control of emitted power of light from the illumination devices in the ToF cameras due to the fact that multiple captures are used to get one frame of data and each capture has to match the other capture very precisely to minimize depth noise. It is noted that even a small change in a signal driving the illumination device (for example, an LED) results into huge changes because of I-V characteristics of the illumination device. In a typical ToF camera, each frame of capture may include multiple quadrants of capture operation. Each quadrant has four phases. In a ‘reset’ phase, a sensor inside the ToF camera is reset to clear accumulated signal from the illuminated objects. In a next phase, ‘integration’, the sensor and illumination are modulated by a time-of-flight controller (TFC) present in the ToF camera, the objects are illuminated and the sensor acquires a raw ToF signal from the illuminated object. In a ‘readout phase’, the raw ToF signal (a raw pixel data) in selected region of interest is readout by an analog to digital converter (ADC) and thereafter by the TFC. In a next phase ‘dead time’, the sensor and the ADC are inactive, and are in low-power mode. In an implementation, ToF assumes that from quadrant to quadrant, a total emitted power of the light emitted by the ToF camera does not change. A variation in total emitted power results into phase/depth error in processing captured images.
For optimal powering of illumination devices, for example LEDs, the ToF camera require specialized regulators that are capable of providing uniform pulse output and minimizing intensity variations, for example, with battery voltage and other operating conditions. Efforts have been made to drive an illumination device present in the ToF camera using analog implantations. For example, the illumination device is controlled by a Direct Current-to-Direct Current (DC-DC) converter, and the DC-DC converter is controlled by a Pulse Width Modulation (PWM) pulse generated by an analog controller. However, in such implementation of analog control of the DC-DC converter, operating points of a loop (formed by the DC-DC converter and the analog converter) tend to change between the quadrants in the ToF camera. Therefore, a digital loop control (by a digital controller) is increasingly used to control the DC-DC converter so as to maintain that the loop's operating point changes only at a frame boundary and not at quadrant boundary. However, unlike an analog loop, a digital loop has only a few fixed possibilities of duty cycle of the PWM pulse, as the duty cycles are changed in form of step changes. Therefore, it is desirable to have a digital based illumination control circuit that is capable of providing finer control of DC-DC converter and thereby a fine control of a constant current drive of the illumination devices.